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that__seth__guy

My Highway Speed Problem

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Hey all, I have a question for you! It seems that on the highway, in my coupe, I am running too high RPM at around 70-80 mph. My speedo says 3500-4000rpm.

 

My setup is a 1971 Ford 302 paired to a Holley 600CFM carb, with a C4 transmission from that year. My rear end is a 1967 Mustang 2.79 ratio. Do I have something in the engine tuned wrong? Timing? Im running 91 octane. Fluid levels in the rear and transmission seem to be fine, but would low transmission fluid level be causing this? Or possibly carburetor tuning? I am in the dark here.

 

 

Any help is, and will always be, greatly appreciated!!!

 

 

Thanks,

 

 

Seth

Edited by that__seth__guy

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I had a 1970 Mustang with 302 and 2.79 rear end years ago. 3,000 rpm in third gear of the C-4 was 80 mph with F70x14 tires which is equal to about 225/70x14 today. Your carb or state of tune should have nothing to due with your issue, could be your tire size or even speedometer error.

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I had a 1970 Mustang with 302 and 2.79 rear end years ago. 3,000 rpm in third gear of the C-4 was 80 mph with F70x14 tires which is equal to about 225/70x14 today. Your carb or state of tune should have nothing to due with your issue, could be your tire size or even speedometer error.

Ah okay, ill have to double check both my Speedo and Tach then. Im running 225/60x15 radials in the rear (Magnum 500's), maybe my setup just isn't a good long-drive highway one?

 

Thanks for your help Bill!

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UPDATE- I did an online calculator and with my tire height of 26.63 inches, I should be turning 2813RPM @ 80MPH, I think my tach may be on the Straight 6 setting as it is an aftermarket tach. *FACEPALM* The sound of the engine may just be off because my exhaust is rusted out behind the seats (new exhaust is on the way to save my hearing). Thanks again Bill for giving me a different perspective and eliminating the carb theory I had. Im still new at this :/

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Another thought is to check out " Tire Rack " for the name and tire size of the tire you are currently running and look at the specs for the tire. They can even tell you the revolutions per mile and such.

Your engine state of tune or fluids will have nothing to do with this but you are running only a three speed transmission... Consider many cars today are running 6 speeds or more.. and that helps keep the RPM down.

Your thoughts on an incorrect tachometer might bear out checking as well. need to be sure it's set for the engine you have.

Good luck with this and maybe post your results when you can.

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Your tach is most likely wrong. I also have a tach that reads too high, and seems to be a % problem, as it is not far off at idle and gets worse at higher RPMs. I am sending mine in for rebuild.

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My highway speed problem is a direct result of my right foot.... :woohoo:

 

 

 

Sorry - couldn't resist!

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My highway speed problem is a direct result of my right foot.... :woohoo:

 

 

 

Sorry - couldn't resist!

:clapping: well put. I applaud you! haha

 

And I will be checking the tach when I get back to the car after Pebble Beach. Ill let you guys know what was going on if/when I fix the issue. Ill also do a photo update of my project, its almost done! :)

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